Established antigen-induced arthritis was produced in 264 out of 339 sensitized Old English rabbits 4-6 weeks after a single intra-articular injection of ovalbumin into one knee joint. Positive arthritis was diagnosed when the joint swelling at this time was greater than or equal to 2 mm. A positive correlation between established joint swelling and delayed hypersensitivity to ovalbumin injected intradermally 24 h prior to joint challenge was demonstrated. Groups of 5-10 arthritic rabbits were dosed orally with increasing or fixed doses of a range of drugs for a period of 3-7 weeks. Drug activity was measured on joint swelling intermittently during the test and on joint macroscopy, cell content of the synovial fluid and joint histopathology at the end of the experiment. Prednisolone sodium phosphate and ketoprofen were the most active compounds tested. Chloroquine diphosphate, cyclo phosphamide, indomethacin, naproxen, SKF 36914 and sudoxicam had some activity, while aspirin, levamisole, oxisuran and D(-)penicillamine had little or no activity.